In the above-described type of devices, the press rolls are formed with liquid permeable shell surfaces, through which liquid is pressed by means of an overpressure, whereby a material web is deposited on the rolls. Liquid treatment, such as washing, is brought about by supplying treatment liquid to the outside of the material web, thereby replacing the original liquid. Finally, the treatment liquid is also pressed out, and in that way the dry matter content of the web is increased. The final dewatering step takes place in the nip, where the dry matter content can amount to between about 35% and 50%.
A space is maintained between the vat and press rolls, through which the material suspension passes. This space converges towards the nip, with disruption in the form of expansion zones where the treatment liquid is supplied. The converging space can also comprise movable baffles extending in the circumferential direction of the rolls.
In order to be able to clean the space between the vat and the press rolls during occasional clogging, the space must be accessible from the outside. This can be effected by rendering the bottom portion of the vat openable, for example it can be vertically adjustable or swivellable. An openable vat bottom, however, creates sealing problems, particularly since the space between the vat and rolls is pressurized in order to facilitate dewatering. Usually a seal in the form of a lip is used, or a seal located between a shoulder on the vat and a corresponding edge on the vat bottom. The vat and vat bottom are held together at that point by means of a bolt connection. These structures, however, do not allow for control and setting of the converging space between the vat and press rolls.
The present invention offers a solution to the aforesaid problems and at the same time renders it possible to open and close the bottom portion of the vat.